Diets high in saturated fats increase low density lipoproteins (LDL) which mediate the deposition of cholesterol on blood vessels. High plasma levels of serum cholesterol are closely correlated with atherosclerosis and coronary heart disease (Conner et al., Coronary Heart Disease: Prevention, Complications, and Treatment, pp. 43-64, 1985). By producing oilseed Brassica varieties with reduced levels of individual and total saturated fats in the seed oil, oil-based food products which contain less saturated fats can be produced. Such products will benefit public health by reducing the incidence of atherosclerosis and coronary heart disease.
The dietary effects of monounsaturated fats have also been shown to have dramatic effects on health. Oleic acid, the only monounsaturated fat in most edible vegetable oils, lowers LDL as effectively as linoleic acid, but does not affect high density lipoproteins (HDL) levels (Mattson, F. H., J. Am. Diet. Assoc., 89:387-391, 1989; Mensink et al., New England J. Med., 321:436-441, 1989). Oleic acid is at least as effective in lowering plasma cholesterol as a diet low in fat and high in carbohydrates (Grundy, S. M., New England J. Med., 314:745-748, 1986; Mensink et al., New England J. Med., 321:436-441, 1989). In fact, a high oleic acid diet is preferable to low fat, high carbohydrate diets for diabetics (Garg et al., New England J. Med., 319:829-834, 1988). Diets high in monounsaturated fats are also correlated with reduced systolic blood pressure (Williams et al., J. Am. Med. Assoc., 257:3251-3256, 1987). Epidemiological studies have demonstrated that the "Mediterranean" diet, which is high in fat and monounsaturates, is not associated with coronary heart disease.
Intensive breeding has produced Brassica plants whose seed oil contains less than 2% erucic acid. The same varieties have also been bred so that the defatted meal contains less than 30 .mu.mol glucosinolates/gram. Brassica seeds, or oils extracted from Brassica seeds, that contain less than 2% erucic acid (C.sub.22:1) and produce a meal with less than 30 .mu.mol glucosinolates/gram are referred to as canola seeds or canola oils. Plant lines producing such seeds are also referred to as canola lines or varieties.
Many breeding studies have been directed to alteration of the fatty acid composition in seeds of Brassica varieties. For example, Pleines and Freidt, Fat Sci. Technol., 90(5), 167-171 (1988) describe plant lines with reduced C.sub.18:3 levels (2.5-5.8%) combined with high oleic content (73-79%). Roy and Tarr, Z. Pflanzenzuchtg, 95(3), 201-209 (1985) teaches transfer of genes through an interspecific cross from Brassica juncea into Brassica napus resulting in a reconstituted line combining high linoleic with low linolenic acid content. Roy and Tarr, Plant Breeding, 98, 89-96 (1987) discuss prospects for development of B. napus L. having improved linoleic and linolenic acid content. Canvin, Can. J. Botany, 43, 63-69 (1965) discusses the effect of temperature on the fatty acid composition of oils from several seed crops including rapeseed.
Mutations can be induced with extremely high doses of radiation and/or chemical mutagens (Gaul, H. Radiation Botany (1964) 4:155-232). High dose levels which exceed LD50, and typically reach LD90, led to maximum achievable mutation rates. In mutation breeding of Brassica varieties, high levels of chemical mutagens alone or combined with radiation have induced a limited number of fatty acid mutations (Rakow, G. Z. Pflanzenzuchtg (1973) 69:62-82).
Rakow and McGregor, J. Amer. Oil Chem. Soc., 50, 400-403 (Oct. 1973) discuss problems associated with selecting mutants affecting seed linoleic and linolenic acid levels. The low .alpha.-linolenic acid mutation derived from the Rakow mutation breeding program did not have direct commercial application because of low seed yield. The first commercial cultivar using the low .alpha.-linolenic acid mutation derived in 1973 was released in 1988 as the variety Stellar (Scarth, R. et al., Can. J. Plant Sci. (1988) 68:509-511). The .alpha.-linolenic acid content of Stellar seeds was greater than 3% and the linoleic acid content was about 28%.
Chemical and/or radiation mutagenesis has been used in an attempt to develop an endogenous canola oil having an oleic acid content of greater than 79% and an .alpha.-linolenic acid content of less than 5%. Wong, et al., EP 0 323 753 B1. However, the lowest .alpha.-linolenic acid level achieved was about 2.7%. PCT publication WO 91/05910 discloses mutagenesis of a starting Brassica napus line in order to increase the oleic acid content in the seed oil. However, the oleic acid content in canola oil extracted from seeds of such mutant lines did not exceed 80%.
The quality of canola oil and its suitability for different end uses is in large measure determined by the relative proportion of the various fatty acids present in the seed triacylglycerides. As an example, the oxidative stability of canola oil, especially at high temperatures, decreases as the proportion of tri-unsaturated acids increases. Oxidative stability decreases to a lesser extent as the proportion of di-unsaturated acids increases. However, it has not been possible to alter the fatty acid composition in Erassica seeds beyond certain limits. Thus, an endogenous canola oil having altered fatty acid compositions in seeds is not available for certain specialty uses. Instead, such specialty oils typically are prepared from canola oil by further processing, such as hydrogenation and/or fractionation.